Daily Arctic Ozone: 1 February 2000 to 30 March 2000 (TOMS) © NASA/GSFC

Waves of energy in the atmosphere are aiding the destruction of Arctic ozone, researchers conclude from satellite temperature measurements. Knowing this should help to predict how climate change might affect ozone depletion in the Arctic.

Arctic ozone thinning, although not as dramatic as the ozone hole that opens up above the Antarctic every spring, exposes plants and animals in the high latitudes of the Northern Hemisphere to more harmful ultraviolet rays from the Sun.

The ozone layer sits in the stratosphere, which is 6 to 30 kilometres above the Earth's surface. Polar ozone destruction is difficult to predict, as it depends on how cold the stratosphere becomes, and this varies each year.

Energy waves in the lower atmosphere can warm the stratosphere, say Paul Newman of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues1. These so-called 'planetary waves' are like the broad slosh of water that moves from one end of the bathtub to the other just as you get out.

Planetary waves are set up by the Earth's rotation in the lower atmosphere (the troposphere). Gradually, they move up to the stratosphere as they encircle the globe from west to east. There they break up and dissipate, like ocean waves reaching the shore. This injects energy into the stratosphere.

So the stratosphere warms up when planetary-wave activity is more intense. Newman's team used temperature measurements made by NASA's Upper Atmosphere Research Satellite to investigate the link between planetary-wave activity and ozone depletion.

Global warming might alter planetary-wave activity in the future

In 1984, for example, when planetary waves were strong and frequent, the Arctic stratosphere didn't become cold enough for significant ozone depletion. In 1997 the waves were weaker and less frequent: "Just right for ozone depletion," says Newman.

Global warming might alter planetary-wave activity in the future. But Newman's observations should make the consequences for ozone loss easier to predict.

Ozone depletion varies with temperature because it relies on the formation of polar stratospheric clouds, which are composed of ice particles. Inert chlorine compounds on the surface of the ice particles are converted to active forms capable of destroying ozone. The chlorine comes mostly from man-made chlorofluorocarbon compounds (CFCs).

In the Antarctic, an isolated vortex of very cold air appears every winter, leading to the formation of stratospheric ice clouds. Ozone-destroying reactions are then triggered by spring sunlight.

The Arctic has a less well-defined polar vortex. There, ozone depletion is subject to the whims of the upper atmosphere - and whether it gets cold enough for stratospheric clouds to form.